Analog-to-digital converter (ADC) with background calibration

Abstract

Analog-to-digital converters (ADCs) with background calibration processes are disclosed. In one aspect, an ADC with a plurality of comparators that each compare an input voltage to voltages that are generated at taps across a plurality of references (e.g., a reference resistor ladder). The comparators are initially calibrated with foreground calibration routines and continuously recalibrated to compensate for aging, voltage, and temperature variations without interrupting operation of the ADC by randomly taking one comparator of the plurality of comparators off-line to run calibration processes without replacing that comparator. The value for the off-line comparator may be reliably inferred from values from neighboring comparators or, in some cases, guessed randomly. While possible errors may be introduced, such errors may be driven to a mean square quantization noise level through exemplary aspects of the present disclosure.

What is claimed is: 
     
         1 . An analog-to-digital converter (ADC) comprising:
 a reference ladder configured to output a plurality of voltage reference levels at a corresponding number of reference outputs;   an input voltage ladder configured to provide an input voltage value at a plurality of nodes equal to the corresponding number of reference outputs;   a plurality of comparators, each coupled to the reference ladder and the input voltage ladder and configured to output a thermometer code value based on signals received from the reference ladder and the input voltage ladder;   a thermometer decoder coupled to the plurality of comparators and configured to assemble the thermometer code values from the plurality of comparators and determine an output value based on how many comparators report a 1 or 0 as the respective thermometer code value; and   a calibration control circuit configured to:
 while in normal operation, randomly take one of the plurality of comparators off-line without providing a substitute comparator; and 
 assemble the thermometer code values from remaining ones of the plurality of comparators and provide the thermometer code value from the off-line comparator by: 
 when comparators above and below the off-line comparator are 1, assign the thermometer code value from the off-line comparator to 1; 
 when the comparators above and below the off-line comparator are 0, assign the thermometer code value from the off-line comparator to 0; and 
 when the comparators above and below the off-line comparator are different, assign the thermometer code value from the off-line comparator randomly to 1 or 0. 
   
     
     
         2 . The ADC of  claim 1 , wherein the calibration control circuit is further configured to, when there is no comparator below the off-line comparator and when the comparator above the off-line comparator is 1, assign the thermometer code value from the off-line comparator to 1. 
     
     
         3 . The ADC of  claim 2 , wherein the calibration control circuit is further configured to, when there is no comparator below the off-line comparator and when the comparator above the off-line comparator is 0, randomly assign the thermometer code value from the off-line comparator. 
     
     
         4 . The ADC of  claim 1 , wherein the calibration control circuit is further configured to, when there is no comparator above the off-line comparator and when the comparator below the off-line comparator is 0, assign the thermometer code value from the off-line comparator to 0. 
     
     
         5 . The ADC of  claim 4 , wherein the calibration control circuit is further configured to, when there is no comparator above the off-line comparator and when the comparator below the off-line comparator is 1, randomly assign the thermometer code value from the off-line comparator. 
     
     
         6 . The ADC of  claim 1 , wherein the reference ladder comprises a plurality of serially-positioned resistors. 
     
     
         7 . The ADC of  claim 1 , wherein the calibration control circuit is further configured to set a duty cycle that controls how often a comparator is taken off-line. 
     
     
         8 . The ADC of  claim 1 , wherein the calibration control circuit is further configured to receive an indication of a receive preamble and take the one of the plurality of comparators off-line during the receive preamble. 
     
     
         9 . The ADC of  claim 8 , wherein the calibration control circuit is further configured to not take a comparator off-line when a receive burst occurs. 
     
     
         10 . The ADC of  claim 1 , wherein the calibration control circuit is further configured to lower a duty cycle of taking one of the plurality of comparators off-line without providing a substitute comparator. 
     
     
         11 . The ADC of  claim 1 , wherein the reference ladder comprises a plurality of references. 
     
     
         12 . A mobile terminal comprising:
 a transceiver circuit comprising an analog-to-digital converter (ADC), comprising:
 a reference ladder configured to output a plurality of voltage reference levels at a corresponding number of reference outputs; 
 an input voltage ladder configured to provide an input voltage value at a plurality of nodes equal to the corresponding number of reference outputs; 
 a plurality of comparators, each coupled to the reference ladder and the input voltage ladder and configured to output a thermometer code value based on signals received from the reference ladder and the input voltage ladder; 
 a thermometer decoder coupled to the plurality of comparators and configured to assemble the thermometer code values from the plurality of comparators and determine an output value based on how many comparators report a 1 or 0 as the respective thermometer code value; and 
 a calibration control circuit configured to:
 while in normal operation, randomly take one of the plurality of comparators off-line without providing a substitute comparator; and 
 assemble the thermometer code values from remaining ones of the plurality of comparators and provide the thermometer code value from the off-line comparator by:
 when comparators above and below the off-line comparator are 1, assign the thermometer code value from the off-line comparator to 1; 
 when the comparators above and below the off-line comparator are 0, assign the thermometer code value from the off-line comparator to 0; and 
 when the comparators above and below the off-line comparator are different, assign the thermometer code value from the off-line comparator randomly to 1 or 0. 
 
 
   
     
     
         13 . A method of calibrating an analog-to-digital converter (ADC), comprising:
 taking one of a plurality of comparators off-line without providing a substitute comparator;   assembling thermometer code values from remaining ones of the plurality of comparators; and   providing a value from the off-line comparator by:
 when comparators above and below the off-line comparator are 1, assigning the value from the off-line comparator to 1; 
 when the comparators above and below the off-line comparator are 0, assigning the value from the off-line comparator to 0; and 
 when the comparators above and below the off-line comparator are different, assigning the value from the off-line comparator randomly. 
   
     
     
         14 . The method of  claim 13 , further comprising taking the one of the plurality of comparators off-line after receipt of a receive preamble indication. 
     
     
         15 . The method of  claim 14 , further comprising not taking the one of the plurality of comparators off-line during a receive burst. 
     
     
         16 . The method of  claim 13 , further comprising imposing a duty cycle on how frequently a comparator of the plurality of comparators is taken off-line. 
     
     
         17 . The method of  claim 13 , further comprising providing the value by, when there is no comparator below the off-line comparator and when the comparator above the off-line comparator is 1, assigning the value from the off-line comparator to 1. 
     
     
         18 . The method of  claim 17 , further comprising providing the value by, when there is no comparator below the off-line comparator and when the comparator above the off-line comparator is 0, randomly assigning the value from the off-line comparator. 
     
     
         19 . The method of  claim 13 , further comprising providing the value by, when there is no comparator above the off-line comparator and when the comparator below the off-line comparator is 0, assigning the value from the off-line comparator to 0. 
     
     
         20 . The method of  claim 19 , further comprising providing the value by, when there is no comparator above the off-line comparator and when the comparator below the off-line comparator is 1, randomly assigning the value from the off-line comparator.